In so-called Lissajous projectors, mirrors oscillating resonantly or almost resonantly and thus sinusoidally in two axes are used. These mirrors, which are also indicated as resonance scanners, are capable of achieving very much greater amplitudes than non-resonantly operated scanners. Greater amplitudes with regard to a scanning laser projection can provide a higher optical resolution.
DE 10 2009 058 762 A1 discloses a deflection device for a projection device for projecting Lissajous figures onto a projection surface, and this is designed, in order to deflect a light beam about at least one first and second deflection axis, for producing the Lissajous figures. The deflection device comprises a deflection unit for producing oscillations about the deflection axes and a control device for producing activation signals for the deflection unit, with a first and a second activation frequency which essentially correspond to the resonance frequencies of the deflection unit, wherein the deflection unit has a quality factor>3000 and the activation device comprises a control loop that is designed to closed-loop control the first and/or the second activation frequency in a manner dependent on the measured phase position of the oscillations of the deflection unit, such that the maximal amplitude of the oscillations remains in the resonance region of the deflection unit. Thereby, the quality factor Q is defined as the ratio of the resonance frequency f0 for the bandwidth B, Q=f0/B, and the bandwidth B with the representation of the amplitude in dependence on the frequency is defined as the width of the resonance peak, at the location, at which the damping reaches 3.01 dB.
Among other things, with the use of resonantly operated scanners of this type, there is the problem of an excess intensity at an edge of the projection surface being able to occur at the reversal points of the oscillation if the beam intensity is temporally integrated with a defined time constant, on account of the minimal speed of the deflection unit. FIG. 4 below, along a line shows an intensity of electromagnetic radiation which is integrated with respect to time, with resonant operation of a resonance scanner with a constant activation frequency (FIG. 4, at the top) corresponding essentially to a resonance frequency of the oscillations, and with a constant maximal amplitude of the oscillations (FIG. 4, centre). One can recognize that the intensity at the end points of the line is increased, whereas the intensity has a minimum in the middle of the line. An application range of such scanners can be restricted due to an intensity increase at the end points of the line.
There exist applications, with which an increased temporally integrated light intensity is not desired at the end points, but at another location, e.g. in the middle of the line or according to an intensity pattern. This e.g. can be the case with illumination tasks, if for instance the centre of the scanned line is of a greater significance than the lateral boundary of the line. However, other applications are directed towards achieving an as homogenous as possible illumination of the line in the temporal mean. A higher intensity at an edge than in the middle of the illuminated surface can also occur in the case of a two-dimensional deflection of a light beam, thus if an area or surface is illuminated.
Scanners that are moved in a quasi-static (non-resonant) manner and at an almost uniform speed are therefore applied in such cases. The problem of not being able to realize large amplitudes of the deflection unit due to the low drives forces which are available, and moreover of a very soft spring suspension of the deflection unit possibly becoming necessary, however occurs with scanners which function in a quasi-static manner. A soft spring suspension however as a rule is very sensitive with regard to shock and vibration and thus has not been able to be meaningfully applied until now for some applications, such as in the field of automobiles.
A very robust scanner with an accordingly hard suspension of the deflection unit however is often necessary for applications with greatly acting vibrations and knocks. Such a scanner in most cases can only generate high amplitudes if it is resonantly operated.